They are part of POSIX so they should work on anything but Windows. This
can be checked simply by a predefined macro, configuration during build
is not necessary. See https://stackoverflow.com/a/4575466 for reference.

Implementation of TimeIndependentPDESolver.

Added timer for GPU synchronization.

- base class with non-virtual destructor causes undefined behaviour
- missing return statements at the end of non-void functions
- missing const before char* function argument
- reordered initialization of class members
- misleading indentation after for statement
- fixed use of uninitialized variables
- fixed unsequenced modification and access to variables (the C++
  standard does not define the evaluation order of operands, so
  expressions with side-effects such as j++ cause undefined behaviour)